In typical telephone systems, phone calls are routed by a central office, through intermediate equipment, and ultimately connected to an individual customer's telephone. Central offices typically serve tens of thousands of telephone lines. Historically, relay circuitry at a central office has made connection of a call, in the form of an analog signal, directly to a pair of copper wires that lead to a called customer's premises. In modernized installations, an increasing portion of those connections are being multiplexed via a digitized link, often fiber optic cable, capable of carrying multiple phone calls through intermediate equipment at a first intermediate terminal. This arrangement removes congestion from the central office. Physical connection of the analog signal to the called customer's copper wire pair can either be made at the first intermediate terminal or at a further intermediate terminal closer to the customer's premises. The further intermediate terminal includes electronic equipment in curbside or in-building pedestals that serve only a few customers. Reducing the distance the analog signal must travel on copper wires to the customer's premises helps preserve signal quality.
The components in such systems are known by various names, but for this discussion the first intermediate terminal is denoted "remote terminal," and the equipment in the pedestal serving a relatively small number of customers is denoted "distant terminal." The copper wires to a customer's premises are referred to as a "loop" or a "line," and the portion of a loop that leads from the street to the house will be called a "drop." Of course, homes and businesses physically close to a central office, need not be coupled through a remote terminal. However, neighborhoods are commonly serviced through remote terminals, each serving about 500 telephone lines. New subdivisions or apartment houses may be served through distant terminals, also called "optical network units," that inwardly communicate to the central office through remote terminals and outwardly serve only a few telephones found in several nearby residences.
Testing of phone lines or drops is an important, conventional function typically provided by automated equipment that sequentially attaches to each phone loop served by a single central office and performs a series of electrical tests on each loop. Central offices with remote terminals present the challenge of performing the electrical tests from the central office on loops that are not attached to the central office. They are attached, often miles away, at the remote terminal. Attachment for testing to a loop not connected to the central office is most commonly accomplished through a single pair of copper wires, called a "bypass pair," that are installed alongside the optical fiber leading to the remote terminal. Thus one pair of copper wires, plus a few spares, provide the means for testing about 500 phone loops extending from a single remote terminal. The same challenge arises in the case of distant terminals, except that the use of one pair of test wires plus a few spares dedicated to testing only the dozen or so phone loops or drops attached at the distant terminal is not efficient. Moreover, the need for installing copper wires defeats one of the purposes of installing a distant terminal that is intended to derive its functionality through optical fibers alone.
The established procedure for testing copper drops at distant terminals has been specified in the Bellcore standard TR-909. That procedure calls for a drop testing device to be installed at each distant terminal. On command, usually nightly, from the automated equipment at the central office, the drop testing device performs a series of tests, analyzes the results, and reports a pass-fail summary of its findings in digitized form via the optical fiber to the remote terminal. The remote terminal, in turn, uses special equipment to receive those digital results and convert them to a selection of resistive values that can be read by the automated equipment at the central office via the bypass pair of wires. The automated equipment at the central office can be programmed to recognize those particular resistive combinations, not as the real resistive condition of a loop, but rather as a special code in which the specific resistive values are indicative of the pass-fail summary.
The specified tests include hazardous voltage (i.e., ac voltage above 135 V or dc voltage above 50 V), foreign voltage (i.e., ac voltage above 10 V or dc voltage above 6 V), resistive faults (i.e., resistance less than 150,000 ohms), receiver off-hook (i.e., a greater than 15% nonlinearity between resistances measured at two different voltages), ringer (i.e., out-of-range ac resistance values, either too high or too low), and sometimes other tests. These tests are measured in various specified combinations across the tip, ring, and the ground terminals, where tip and ring are the names of the two wires in the loop or drop leading to a telephone. Additionally, if multiple tests fail, only one failure is reported based on a predetermined priority. Signals are not provided that can be analyzed to derive specific values for test results.
The described test equipment is inefficient in the sense that each unit serves to test only a few phone drops at a single distant terminal and is used at a particularly low duty cycle, e.g., a few seconds each night. Additionally, the test circuit is complex, e.g., it includes functions of connecting, scaling, rectifying, filtering, measuring, calibrating, analyzing, prioritizing, and reporting. It only generates a single pass-fail report with no proportional data that might be useful for identifying a degrading system or a system with multiple failures. Moreover, the system defined in TR-909 only tests the phone drops themselves, and does not provide for evaluation of optical communication components and the fiber optic cables between the remote terminal and the distant terminal. The system interrogates at a particular time, and then a test is performed. A test in a nominal system may take 35 seconds to perform per line but only one half second to transmit. When tests are made only at the time a customer telephone is interrogated, there is a chance of encountering a busy condition. If test results can be obtained at any time during the day and transmitted upon receipt of an interrogation signal, the busy condition can be avoided. In summary, the distant terminal test circuits are expensive per line tested and do not provide the comprehensive testing that could be more useful for service improvement and reliability.